The need for using miniature and high performance filters in wireless communication devices has led to the widespread usage of Surface Acoustic Wave (SAW) filters. In addition to Surface Acoustic Wave (SAW) filters Bulk Acoustic Wave (BAW) filters can also be used. Bulk Acoustic Wave (BAW) filters typically include several Bulk Acoustic Wave(BAW) resonators. In a Bulk Acoustic Wave (BAW) filter, acoustic waves propagate in a direction that is perpendicular to the filter's layer surfaces. In contrast, acoustic waves which propagate within a Surface Acoustic Wave (SAW) filter do so in a direction that is parallel to the layer surfaces of the filter.
It is known to fabricate monolithic filters that include at least a Bulk Acoustic Wave (BAW) resonator device (also known in the art as “Thin Film Bulk Acoustic Wave Resonators (FBARs)”). By example, Bulk Acoustic Wave (BAW) resonators typically include two electrodes and a single piezoelectric layer that is deposited between the two electrodes. One or more acoustic isolation layers may also be employed between the piezoelectric layer and a substrate of the respective devices.
Bulk Acoustic Wave (BAW) filters can be fabricated to include various known types of Bulk Acoustic Wave (BAW) resonators. These known types of Bulk Acoustic Wave (BAW) resonators comprise three basic portions. A first one of the portions, which is used to generate acoustic waves, includes an acoustically-active piezoelectric layer. This layer may comprise, by example, zinc-oxide(ZnO), aluminum nitride(AIN), zinc-sulfur (ZnS), or any other suitable piezoelectric material that can be fabricated as a thin film. A second one of the portions includes electrodes that are formed on opposite sides of the piezoelectric layer. A third portion of the Bulk Acoustic Wave (BAW) resonator includes a mechanism for acoustically isolating the substrate from vibrations produced by the piezoelectric layer. Bulk Acoustic Wave (BAW) resonators are typically fabricated on silicon, gallium arsenide, or glass substrates using thin film technology (e.g., sputtering, chemical vapor deposition, etc.). Bulk Acoustic Wave (BAW) resonators exhibit series and parallel resonances that are similar to those of, by example, crystal resonators. Resonant frequencies of Bulk Acoustic Wave (BAW) resonators can typically range from about 0.5 GH to 5 GHz, depending on the layer thicknesses of the devices.
FIG. 1 shows an example of a Bulk-Acoustic-Wave (BAW) resonator using an acoustic mirror to acoustically isolate the resonator from the substrate. The Bulk Acoustic Wave (BAW) resonator 20 comprises a substrate 10 having a top surface 12 and a bottom surface 14. Acoustic mirror 31 overlies the top surface 12 of the substrate. The resonator further comprises a piezoelectric layer 22 interposed between a first electrode 21 and a second electrode 23, and a protective layer 16. The piezoelectric layer 22 comprises, by example, a piezoelectric material that can be fabricated as a thin film such as, by example, zinc-oxide (ZnO), or aluminum nitride (AlN).
In the example shown in FIG. 1 the acoustic mirror 31 comprises three layers, namely a top layer 31a, a middle layer 31b, and a bottom layer 31c. Each layer 31a, 31b and 31c has a thickness that is, by example, approximately equal to one quarter wavelength of the resonance frequency of the resonator. The top layer 31a and bottom layer 31c are made of materials having low acoustic impedances such as, by example, silicon oxide(SiO2), poly-silicon, aluminum(Al), or a polymer. Also, the middle layer 31b is made of a material having a high acoustic impedance such as, by example, gold (Au), molybdenum (Mo), or tungsten (W). The ratio of the acoustic impedances of consecutive layers is large enough to permit the impedance of the substrate to be transformed to a lower value. As a result, the substrate 10 may be comprised of various high acoustic impedance materials or low acoustic impedance materials (e.g., Si, SiO2, GaAs, glass, or a ceramic material). The number of layers in an acoustic mirror can vary broadly depending on the degree of acoustic isolation required for the respective filter device. Usually three to up to nine layers are used, wherein uneven and even numbers of layers are possible.
In FIG. 2 an example of a BAW device is shown which comprises a Stacked-Crystal-Filter (SCF) on a substrate 10.
The Stacked Crystal Filter (SCF) 50 comprises a lower electrode 21, a middle electrode 23, and a top electrode 25.
Interposed between the lower and the middle electrode is a first piezoelectric layer 22. Interposed between the middle and the upper electrode is a second piezoelectric layer 24.
The piezoelectric layer 22 comprises, by example, a piezoelectric material that can be fabricated as a thin film such as, by example, zinc-oxide(ZnO), or aluminum-nitride(AlN). The second piezoelectric layer 24 may comprise similar materials as the first piezoelectric layer 22. The middle electrode 23 is usually employed as a ground electrode. The top electrode 25 may comprise similar materials as the bottom and middle electrodes 21 and 23, for example Al.
The solidly-mounted Stacked Crystal Filter 50 shown in FIG. 2 comprises an acoustic mirror 31 which acoustically isolates vibrations produced by the piezoelectric layers 22 and 24 from the substrate 10. The acoustic mirror 31 shown in FIG. 2 also comprises three layers, namely a top layer 31a, a middle layer 31b, and a bottom layer 31c. Each layer 31a, 31b and 31c has a thickness that is, by example, approximately equal to one quarter wavelength of the resonance frequency of the resonator. The top layer 31a and bottom layer 31c are made of materials having low acoustic impedances such as, by example, silicon oxide(SiO2), poly-silicon, aluminum (Al), or a polymer. Also, the middle layer 31b is made of a material having a high acoustic impedance such as, by example, gold (Au), molybdenum (Mo), or tungsten (W). It should be noted that a membrane or tuning layer (not shown) may also be provided between the acoustic mirror 31 and the electrode 21 of the device 50, if needed for tuning the device 50 to enable it to provide desired frequency response characteristics.
A problem encountered with solidly mounted Bulk-Acoustic-Devices is that the acoustic isolation of the resonator by the acoustic mirror is not complete and that therefore a part of the acoustic energy leaks into the substrate and is reflected from the bottom surface of the substrate back up to the resonator. This phenomena causes ripples in the filter's passband, deteriorating it's performance. For some frequencies, depending on the thickness of the substrate, the substrate may even form an acoustic cavity, which increases the negative effects on the resonators.
In order to lessen these deteriorating effects, it has been proposed to use substrate materials which have a high absorbency for the acoustic waves in question. Therefore, most of the energy of the acoustic wave which leaked into the substrate is absorbed before the wave reaches the bottom surface of the substrate. A suitable material is for instance glass. The problem with this solution is that glass substrates are not compatible with standard CMOS production processes.
Alternatively it has been suggested to increase the number of mirror layers or to use a very thick acoustic mirror made of tungsten to decrease the leakage of acoustic energy into the substrate. Unfortunately, both these solutions cause considerable additional costs in the production of such devices.
Another approach to avoid this problem is to use bridge type BAW-resonators in the filter devices. Such resonators use an air gap underneath the resonator to acoustically isolate it. However, the costs for fabricating such bridge-type BAW resonators is much higher than for those using acoustic mirrors. Furthermore, the use of bridge-type resonators puts further constraints regarding suitable packaging of such filter devices.